Selection Flashcards
What are the evolutionary problems the brain is trying to solve?
Survival and Reproduction
Specific functional system
one whose effects can be traced to a distinct location.
Example of a system with a specific function
The superior colliculus: Visual information is received from the retina, and processed by a somatotopic map, which directly influences eye movement
Generic function system
structures with repeating microarchitecture that process information from regions that perform widely different functions. All systems with a general function consist of reciprocal connects (re-entrant loops) with other regions.
Example of a system with a general function
the cerebellum
- has re-entrant loops with sensorimotor, cognitive, and motivational structures which allow the calibration and refinement of movement.
- It also possesses a repeated microarchitecture whereby the basic structural organisation is uniform across the cerebellum, despite having differing input/outputs.
What are two broad generic functions provided by the basal ganglia
- selection of the most appropriate movement, sensation, idea, emotion, etc… in a given situation based on previous experience
- reinforcement learning, whereby strategies are learnt that maximise the chance of a successful outcome in a given situation
How is the architecture of the basal ganglia relevant to its different functions?
The basal ganglia is functionally separated:
- Medial/Anterior = Limbic (NA/Caudate)
- Central = Associative (central striatum)
- Lateral/Posterior = Sensorimotor (putamen)
What are the major cell types/compositions within the striatum?
- 97% MSNs (medium spiny neurons): only cell type that projects out of the striatum.
- > 1% Cholinergic interneurons: receive thalamic and some cortical inputs and have a great influence on the function of the basal ganglia
- GABAergic interneurons
- VTA/SNpc dopaminergic neurons
What do the functional regions of the BG differ in?
o External inputs (cortex, limbic system, brainstem)
o Input functions (motivation, mood, cognition, sensorimotor)
Why is a brain area tasked with selection so important?
Our brain needs to address competing motivations (food, escape, fluid balance, pleasure) with the single set of muscles. Having a region specialised for selection allows relative importance of each motivation in order to determine which actions are executed.
Describe the disinhibitory loop of the BG
- The cortex has excitatory connections onto the MSNs of the striatum.
- MSNs have inhibitory projections onto SNpr neurons.
- The SNpr neurons tonically inhibit the thalamus.
- The thalamus has excitatory connections with the cortex.
A phasic excitatory burst from the cortex results in increased inhibition of SNpr neurons by increased MSN activity, which disinhibits the thalamus, allowing it to excite the cortex.
Mutual inhibition
The increased stimulation of a MSN by the cortex results in inhibition of nearby MSNs, to allow selection of an output channel.
Reinforcement learning
Reinforcement learning describes how actions that maximise the likelihood of a successful outcome are likely to be selected for.
Describe two targets where reinforcement can bias selection
- Cortical excitability
- Corticostriatal plasticity
What does dopamine signal?
Dopamine acts to signal the difference between and expected outcome and the actual outcome.
Describe an experiment to test the two modes of dopamine release on corticostriatal plasticity.
Experiment
Set up: Obtain an ex vivo slice from the thalamus/cortex that contains the desired circuitry
+ Insert stimulating electrode into the cortex and dopaminergic cells, as well as recording electrode on the MSNs
1. Stimulate the cortex on its own to obtain the baseline post-synaptic potential in MSNs
2. Co-stimulate dopamine cells and glutaminergic cortical cells
(Do this tonically/phasically to represent the absence/presence of a reward)
3. Stimulate the cortex on its own to determine if post-synaptic potential has increased/decreased
